As is known in the art, in telecommunications systems, satellite systems and other systems, it is desirable for radio frequency (RF) power amplifiers (PAs) to linearly amplify RF signals in a highly efficient manner. Efficiency is generally proportional to input drive level. High efficiency is typically not attained until an amplifier approaches its maximum output power. This, however, is not consistent with linear operation. Thus, a tradeoff must typically be made between achieving maximum efficiency and high linearity in RF power amplifier circuits.
As is also known, so-called Doherty-type amplifiers or more simply Doherty amplifiers have been used to overcome this problem. Generally, a Doherty amplifier includes a pair of transmission paths connected in parallel between a source and a load. Each of the transmission paths includes an amplifier.
In one signal path, the amplifier is provided as a class "B" or a class "AB" amplifier (also referred to as "carrier amplifier" in the Doherty amplifier design). The carrier amplifier is designed and biased to amplify signals having relatively low signal levels. In the other signal path, the amplifier is provided as a class "C" amplifier (also referred to as a peak amplifier in the Doherty amplifier design). The peak amplifier is designed and biased such that it is off when the instantaneous value of the RF signals provided by the signal source have a signal level below a predetermined threshold. When the signal level of the RF signal fed to the peak amplifier input port reaches a predetermined threshold level, the peak amplifier is biased on and both the peak and carrier amplifiers deliver RF power to the load.
With this approach, the Doherty amplifier is able to provide an optimum power added efficiency (PAE) over a desired range of amplifier output power (P.sub.out). An ideal Doherty amplifier has a constant PAE value over a 6 decibel (dB) range of P.sub.out.
The carrier and peak amplifiers provided in the parallel signal paths of the Doherty amplifier may be built with high-power field effect transistors (FETs). In general, the optimum load impedance of a high power FET is relatively low at high power levels. This makes it relatively difficult to combine the signals from the carrier and peak amplifiers while at the same time maintaining a relatively high gain and PAE over a range of Doherty amplifier output power.
It would, therefore, be desirable to provide an RF power amplifier that provides a desired PAE over a relatively wide range of amplifier output power. It would also be desirable to provide an RF power amplifier that is linear and efficient for multi-carrier noise-like signals.